An article conveying system used in a production facility of semiconductor devices is sometimes configured such that carriers (overhead carriers) traveling along transport tracks (traveling rails) provided near the ceiling of the facility carry suspended articles (such as magazines, FOUPs, or cases that accommodate multiple semiconductor wafers or semiconductor devices during production) so as to transport the articles along the transport tracks.
A typical overhead carrier used in such an article conveying system has wheels (driving wheels) on the right and left of a vehicle body, for allowing the overhead carrier to travel on a pair of right and left traveling rails. In many cases, an axle for transmitting a rotary force to the right and left driving wheels is shared by the right and left wheels. In this case, a traveling distance of the driving wheels is different between the right and left driving wheels (turning radius differentials arise) in a curved section of the transport track, that is, a section where the traveling rails are curved. Thus, the posture of the overhead carrier is laterally deviated from the traveling direction of the curved section in the case that the overhead carrier travels as in a linear section. In order to prevent the overhead carrier from being caught and stopped in the curved section or falling from the traveling rails in the worst case, measures have to be taken against turning radius differentials, for example, an adjustment to the number of revolutions of the right and left driving wheels by means of a differential gear.
However, in addition to a control unit etc. for suspension of articles, the provision of complicated mechanisms such as a differential gear for making a difference in the number of revolutions between the right and left driving wheels may excessively increase the size of the overhead carrier. This may require a large transportation space and increase the manufacturing cost of the overhead carrier.
Against this backdrop, in an invention described in Japanese Patent Laid-Open No. 2008-044400, the driving wheel of a carrier includes a large-diameter portion and a small-diameter portion. The small-diameter portion in a curved section comes into contact with a traveling rail on the inner track, whereas the large-diameter portion in the curved section comes into contact with a traveling rail on the outer track. Thus, the right and left driving wheels driven with a common drive shaft can smoothly travel in the curved section without using mechanisms such as a differential gear.
Unfortunately, the carrier described in Japanese Patent Laid-Open No. 2008-044400 can only respond to a curve with a radius of curvature corresponding to the diameter ratio between the large-diameter portion and the small-diameter portion.
In other words, even if the carrier can smoothly travel along a curve with a specific radius of curvature, the carrier may have a turning radius differential along a curve with other radius of curvature, leading to unsmooth traveling. This limits a radius of curvature usable for the curved section to one specific radius of curvature, resulting in a restricted layout of the traveling rail.
Moreover, in the article conveying system including the overhead carrier, a mechanism for suspending an article may collide with the traveling rail on the outer track in the curved section depending on the shape of the mechanism. In other words, the mechanism moves over a large area on the outer track of the curved section and thus may collide with the traveling rail in the case that the traveling rail exists within the range of movement of the mechanism.
To prevent this problem, the traveling rail may not be provided on the outer track in a range where a collision may occur, so as to provide a section where the traveling rail is cut on the outer track.
In the section where the traveling rail is cut on the outer track, however, only the inner wheel comes into contact with the traveling rail. Thus, a starting torque needs to be obtained only by the inner wheel in preparation for stop of traveling of the overhead carrier in this section.
An object of the present invention is to provide an overhead carrier that can travel on curves with various curvatures so as to increase the layout flexibility of traveling rails, and obtain a torque for restart of the overhead carrier stopped in any section while preventing the mechanism of the overhead carrier from colliding with the traveling rail.